Optical elements (usually made of glass) that utilize lens diffraction are commonly used to enhance optical performance, and particularly to reduce aberration and enhance picture quality (see Patent Document 1, for example). In an optical system, the degree of freedom in aberration correction must be increased in order to adequately reduce Seidel's five aberrations and chromatic aberration with respect to a reference spectral line, and the number of lenses in the optical system often unavoidably increases.    Patent Document 1: U.S. Pat. No. 6,130,785